Wavelength multiplexers are key building blocks for WDM communication networks that use multiple wavelengths on a single optical fiber. Integrating the wavelength multiplexer with either transmitters or receivers on a single chip allows cost saving and leads to smaller components.
The most commonly used building block to realize such a multiplexer is an arrayed waveguide grating (AWG). The basic layout of an AWG is schematically shown in FIG. 1 that is taken from a prior art document by M. K. Smit and C. van Dam, “PHASAR-Based WDM-Devices: Principles, Design and Applications”, IEEE J. of Sel. Top. In Quant. Electr., Vol. 2, No. 2, June 1996.
FIG. 1 shows that the AWG 1 has an input slab region or input free propagation range (FPR) 2 and an output slab region or output FPR 3 that are in optical communication by an array of waveguides 4 having a length increment from one waveguide to the next. Furthermore, the blow up in FIG. 1 schematically shows the arrangement of the waveguides of the array of waveguides 4 at the input section of the output FPR 3 of the AWG 1.
Furthermore, it can be seen from FIG. 1 that the output slab 3 region of the AWG 1 is also in optical communication with a plurality of output waveguides 5, whereas the input slab region 2 is also in optical communication with a first input waveguide for receiving a WDM optical input signal. The AWG 1 further has a central channel wavelength, λc, and a number of channels that are spaced apart by a channel spacing, ΔλAWG.
Only when the waveguides 4, 5, 6 that compose the AWG 1 are perfectly polarization insensitive, which means that for the two main polarization states, i.e. transverse electric (TE) and transverse magnetic (TM), the effective mode indices are exactly the same, i.e. Neff,TE=Neff,TM, the transmission response is reasonably similar for both these polarization states. In the case that Neff,TE≠Neff,TM, the waveguides are said to be birefringent. As a result thereof, the AWG 1 has a different transmission response for different TE and TM polarized input light.
Known polarization insensitive wavelength multiplexers comprise stand alone AWG components that are usually made in silica technology that enables the waveguides of the stand alone AWG to be made polarization insensitive with reasonable fabrication tolerance. However, the silica platform is not very suitable for densely integrated circuits on chip as it is not possible to monolithically integrate transmitters, receivers and/or modulators in the same platform. Therefore, the use of the silica platform is disadvantageous because of the rather bulky stand alone components and the required hybrid integration thereof. This results in higher complexity and costs of systems fabricated using this technology.
There are several material systems that are more suitable than the silica platform to establish smaller components and easier integration thereof into complex systems such as multi-wavelength transmitters and receivers. Examples are the known Indium Phosphide (InP) system and the Silicon On Insulator (SOI) platform. A drawback of using these known material systems is that the standard waveguide types in both these material systems have substantially different effective mode indices for the two polarization states. This results in a shift of the wavelength response between the two polarizations TE and TM. This is called polarization splitting, polarization dispersion or polarization dependent wavelength shift (PDWS). The relative PDWS (Δλ/λ) is equal to the relative mode index difference (ΔNeff/Neff):
                              Δλ          λ                =                              Δ            ⁢                                                  ⁢                          N              eff                                            N            eff                                              Eq        .                                  ⁢        1            
FIG. 2a schematically shows different orders of TE and TM polarized light, i.e. TEm, TEm-1, TMm, and TMm-1, that exit the waveguides of the array of waveguides 4 that are arranged at a first side of the output slab region 3 of the AWG 1 are projected on the output waveguides 5 that are arranged at the opposite side of the output slab region 3 of the AWG 1.
FIG. 2b schematically shows a spectral distribution of the different orders of the TE and TM polarized light, i.e. TEm, TEm-1, TMm, TMm-1. From this spectral distribution the polarization dispersion (pd) and the Free Spectral Range (FSR) can be determined.
From the above mentioned prior art document by M. K. Smit and C. van Dam, an approach is known for making an AWG polarization independent. In this approach two separate inputs for the two polarization states, i.e. TE and TM, are used as is schematically shown in FIG. 5. In this case, the input slab region 2 of the AWG 1 is further in optical communication with a second input waveguide 7. The first input waveguide 6 is provided with TE polarized light from the optical input signal, whereas the second input waveguide 7 is provided with TM polarized light from the optical input signal.
The first 6 and second 7 input waveguides are arranged relative to the input slab region 2 at a first and a second position. These first and second positions are matched to the polarization dispersion caused by the birefringence of the waveguides of the array of waveguides 4 of the AWG 1.
In the known approach mentioned above, a polarization beam splitter 8 for splitting the TE and TM polarization states of the optical input signal is required in combination with the birefringent AWG 1.
Integrated polarization beam splitters are known from Y. Hashizume et al., “Integrated Polarisation Beam Splitter using Waveguide Birefringence Dependence of Waveguide Core Width”, Electr. Lett. Vol. 37, No. 25, December 2001, and from US Patent Application 2009/0214150A1.
The integrated polarization beam splitter known from Y. Hashizume et al. has a disadvantage in that it is silica-based and therefore not suitable for monolithic integration with transmitters, receivers and/or modulators. Although the disclosed polarization beam splitter would be suitable for a WDM system because of its broad bandwidth, it has another disadvantage in that its performance is very sensitive to deviations in the fabrication process as its polarization splitting response is substantially determined by a difference in birefringence between the two arms of the Mach-Zehnder Interferometer-based polarization beam splitter.
Although US 2009/0214150A1 discloses a polarization beam splitter that is integrated using the InP material system, a disadvantage of this polarization beam splitter is that it is not suitable for use in a wavelength multiplexer for a WDM system because of its very narrow bandwidth that would only allow it to be used for one channel of the WDM system.